One of the things that struck me in looking at superionic water is how it is so often spoken of as though it were partly liquid and partly solid. I did a spot of reflection, which I am thinking it may help to headline. Where, some of the ideas being brought up will help us on the onward subject of looking at how memristors work.

So, following up on the new form of water post:

KF, 4: >>One of the interesting things about the coverage [on superionic water] is how they phrased the properties of superionic ice in terms of being both a solid and a liquid.

This may suggest to the public the notion of a contradictory state, which in my view is probably not very helpful. Mind you, solid and liquid is a somewhat fuzzy border as there are things that blend properties: toothpaste and ketchup or some kinds of paint being familiar examples — resisting flow to some extent then yielding and “going” all at once.

[Let me add a nice illustration that shows how this can work, in the context of a substance being thixotropic:]

Now, too, metals can be solid [think, Iron or Aluminum under typical conditions] or liquid (think, Mercury or “molten metals”) Where, the key property of conduction band — “free” — electrons or the like is independent of the two states; this pattern of “free” electrons is what makes many metals shiny and good conductors of electricity or heat. There are of course complexities such as why some metals are yellow or reddish. In the solid state, we have a lattice of atoms packed together in a fairly fixed pattern, which is why solid metals have a given volume and hold a given shape. Alloys, of course blend in other atoms, and this affects the properties in significant and technologically useful ways. E.g. that is how we get the family of alloys we know as steel.

We also have liquids, which tend to have an almost-solid, distorted structure with holes or gaps; which allows for flow and the property of a definable volume but inability to support itself against shear thus flow. Indeed, the point where we recognise liquidity is when we spot that a body of material cannot support itself against its own weight and so flows or spreads and levels its surface. But, it has a more or less fixed volume. Linked, we have degree of viscosity; which shapes the rate of such flow.

Glasses can be viewed as super-viscous liquids that flow at negligible rates.

Gases or vapours — the classic inter-atomic bonding curve applies —

have sufficient separation of molecules that they lack the structuring of fairly strong inter-molecular bonds and so do not have a specific volume.

A plasma is an ionised gas, often in the context of high temperatures. Sparks, lightning strokes and the gas bodies of stars are fairly familiar examples.

Superionic ice should have metal-like and plasma-like properties, perhaps also properties comparable to molten salts, which are ionic crystals. If protons (H+, not neutral H atoms) are roaming, there will be ionic structures in the lattice simply on matter and charge conservation. These will be modified by the fact that the proton is over 1800 times the mass of an electron.

Wiki’s discussion of superionic water is interesting:

Superionic water is a phase of water under extreme heat and pressure which has properties of both a solid and a liquid, which is supported by some experimental evidence [1][2][3][4]

At high temperatures and pressures, such as in the interior of giant planets, it is argued that water exists as ionic water in which the molecules break down into a soup of hydrogen and oxygen ions. At even higher pressures, ionic water will further condense into superionic water, where the oxygen crystallises and the hydrogen ions float around freely within the oxygen lattice.[5] . . . .

Superionic water was previously theoretical, but predictions were made about its properties. If it were present on Earth, it would rapidly decompress and explode. Under the conditions theorized to cause water to enter the phase, it is believed that superionic water would be as hard as iron and would glow yellow.[4]

As of 2013 it is theorized that superionic ice can possess two crystalline structures. At pressures in excess of 0.5 Mbar [–> 1 bar is 100,000 Pascals] it is predicted that superionic ice would take on a body-centered cubic structure. However, at pressures in excess of 1 Mbar it is predicted that the structure would shift to a more stable face-centered cubic lattice . . . .

Demontis, et al. made the first prediction for superionic water using classical molecular dynamics simulations that were published in Physical Review Letters in 1988 [7]. In 1999 Cavazzoni, et al. predicted that such a state would exist for ammonia and water in conditions such as those existing on Uranus and Neptune.[8]. In 2005 Laurence Fried led a team at Lawrence Livermore National Laboratory in California to recreate the formative conditions of superionic water. Using a technique involving smashing water molecules between diamonds and super heating it with lasers they observed frequency shifts which indicated that a phase transition had taken place. The team also created computer models which indicated that they had indeed created superionic water.[4] In 2013 Hugh F. Wilson, Michael L. Wong, and Burkhard Militzer at the University of California, Berkeley published a paper predicting the face-centered cubic lattice structure that would emerge at higher pressures.[6]Additional experimental evidence was found by Marius Millot and colleagues in 2018, in an article in Nature Physics, by inducing high pressure on water between diamonds and then shocking the water using a laser pulse.[2].

The 2004 Phys Rev Letters abstract [Wiki’s Ref 1] summarises:

Raman spectroscopy in a laser heated diamond anvil cell and first principles molecular dynamics simulations have been used to study water in the temperature range 300 to 1500 K and at pressures to 56 GPa. We find a substantial decrease in the intensity of the O-H stretch mode in the liquid phase with pressure, and a change in slope of the melting line at 47 GPa and 1000 K. Consistent with these observations, theoretical calculations show that water beyond 50 GPa is “dynamically ionized” in that it consists of very short-lived (<10 fs) H2O, H3O+, and OH- species, and also that the mobility of the oxygen ions decreases abruptly with pressure, while hydrogen ions remain very mobile. We suggest that this regime corresponds to a superionic state

So, beyond 50 GPa [= 1/2 a million bars, 0.5 Mbar], we have emergence of plasma-like conditions or comparable to a molten ionic crystal, though separations are of order of fast chem reaction times, ~ 10^-14s. As pressure rises, the O atoms are squashed together and shift to the locked-in lattice pattern. Where, the two metallic crystal structures point to more or less close packing of spheres, i.e. this is not directional bonding. Close-packing of spheres is a characteristic pattern of metals. It looks like the larger O ions are being squashed together to the point where their electronic clouds mutually repel, forming a metal-like packing pattern. O atoms are of 10^-10 m scale, protons of 10^-15 m scale.

The H+ ions then retain mobility as charge carriers, as they have not been squashed in close enough for nuclear force level interaction. There is plenty of room in the lattice.

It seems this is what was recently observed.

However, “abrupt” loss of mobility for O is another way of saying, phase change without fully acknowledging it. The result is now a solid, though perhaps squishy — by comparison, many metals are soft. Reasonably pure iron is not a particularly hard metal, notwithstanding the suggestion “as hard as” above. Steels and cast iron have admixtures of C and perhaps other elements.

A Physics dot org 2005 article may help us see more:

While everyone is familiar with water in the liquid, ice, and gas phases, water can also exist in many other phases over a vast range of temperature and pressure conditions. One lesser known phase of water is the superionic phase, which is considered an “ice” but exists somewhere between a solid and a liquid: while the oxygen atoms occupy fixed lattice positions as in a solid, the hydrogen atoms migrate through the lattice as in a fluid. Until now, scientists have thought that there was only one phase of superionic ice, but scientists in a new study have discovered a second phase that is more stable than the original. The new phase of superionic ice could make up a large component of the interiors of giant icy planets such as Uranus and Neptune.

The highlighted seems to be close to the root of the solid and liquid view. To this, I suggest that if the O atoms are in a lattice, we have the basis for solidity. though it may be squishy. H+ ions flowing around are then more analogous to the conduction band electrons of a metal, but they are much heavier. I suspect, the context that we start with water and this involves H atoms bonded to O leads to the fairly awkward phrasing in terms of superposition of solid O and liquid H.

The conception of the inner structure of metals [there is much more than I have mentioned!] is of course not a commonplace household idea. The further point that metals can be solid or liquid does not help matters.

I suggest, discussing the superionised phase in terms of the lattice and the “free” charge carriers.

Where, there may be some mobility so that we may have properties of a Bingham or of a Thixotropic fluid . . . resisting flow up to a point, then yielding and flowing much like toothpaste or ketchup . . . then those sorts of intermediates may be brought in. But then, under pressure in our home planet’s crust and mantle, we do get flowing of what we would normally consider solids.

We are getting into fairly unusual and complicated territory here.

A take-home point should be that the borderline between a solid and a liquid is somewhat fuzzy and all sorts of things may be happening along that border. Another, is that molecular structure counts.>>

Food for thought, and getting us in the groove for discussing the vexed question, how do memristors work. END

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